Recent automobiles have been equipped with numerous pieces of electric equipment such as air conditioners, audio units, power seats, and power windows. The power consumption tends to increase due to such an electric load. Therefore, the electric current flowing through the contacts for the key switch that controls the supply of electric power to such onboard electric equipment tends to be increased. In particular, activation of the starter motor involves a substantial amount of electric current, which results in significant heat generation from the contacts of the key switch. Such heat generation is not desirable because it accelerates the deterioration of the plastic materials used for the holder members and the couplers that secure the contact terminals.
To control the heat generation of the contacts, it is necessary to increase the heat capacity of the contacts, but, because such contacts are generally enclosed for the needs of electric insulation the generated heat cannot be readily removed. When an attempt is made to increase the heat capacity of the contacts, the size of the key switch inevitably becomes larger.